It is common for the devices of a building management system to communicate using a token ring network, such as a network using the BACnet™ protocol. In a token ring network, a token is passed in turn among the devices on the network, with a device only being allowed to transmit on the network when it is in possession of the token.
Traditionally, the speed of communication between devices in a building management system has not been of particular importance, and consequently networks such as BACnet™ have not been designed to ensure fast communication. However, modern building management systems may now include devices controlled by control devices, for which a user will expect an immediate response, requiring that the communication time between the devices is sufficiently small. For example, a user may be able to control a set of electric window blinds using a control panel, and in this case a significant delay between pushing a button on the control panel and the blinds taking the required action will cause annoyance to the user.
A medium-sized building management system may comprise 64 devices that can communicate via the network. If the average delay in passing the token between devices in the network is 4.5 ms, which is not unusually slow, this leads to a time taken for the token to complete a circuit of the devices of the network of 290 ms. In addition, a control device might take 50 ms to acquire an input signal in response to a user pressing a button on a control panel, and 75 ms to execute internal logic to produce a message to send over the network. A device receiving the message then may take 75 ms to execute internal logic and then take the required action. This results in a potential delay of the order of 400 ms, and more for building management systems with more devices, which is sufficiently long to cause annoyance to a user. Ideally a maximum potential delay would be 250 ms, giving a maximum time for communication between devices of 50 ms.
The present invention seeks to mitigate the above-mentioned problems. Alternatively and/or additionally, the present invention seeks to provide improved methods and devices for communicating over a building management system network.